1. Field of the Invention
The present invention pertains to processing gravity and magnetic data using vector and tensor data along with seismic data and more particularly to the inversion of gravity and magnetic data and combining the result of the inversion process with seismic data to provide velocity models and to improve depth models to locate possible hydrocarbon bearing zones in areas of anomalies such as salt, or igneous formations.
2. Related Prior Art
Exploration for hydrocarbons in subsurface environments containing anomalous density variations has always presented problems for traditional seismic imaging techniques by concealing geologic structures beneath zones of anomalous density. Many methods for delineating the extent of the highly anomalous density zones exist.
U.S. Pat. No. 4,987,561 , titled xe2x80x9cSeismic Imaging of Steeply Dipping Geologic Interfaces,xe2x80x9d issued to David W. Bell, provides an excellent method for determining the side boundary of a highly anomalous density zone. This patent locates and identifies steeply dipping subsurfaces from seismic reflection data by first identifying select data which has characteristics indicating that the acoustic pulses which it represents have been reflected from a substantially horizontal or steeply dipping interface. These data are analyzed and processed to locate the steeply dipping interface. The processed data are displayed to illustrate the location and dip of the interface. This patent, while helping locate the boundaries, provides nothing to identify the subsurface formations on both sides of the boundary.
There have also been methods for identifying subsurface formations beneath anomalous zones using only seismic data to create a model and processing the data to identify formations in light of the model. By further processing acoustic seismic data, the original model is modified or adjusted to more closely approximate reality.
An example of further processing seismic data to improve a model is U.S. Pat. No. 4,964,103, titled xe2x80x9cThree Dimensional Before Stack Depth Migration of Two Dimensional or Three Dimensional Data,xe2x80x9d issued to James H. Johnson. This patent provides a method of creating a three-dimensional model from two dimensional seismic data. This is done by providing a method of ray tracing to move before stack trace segments to their approximate three-dimensional position. The trace segments are scaled to depth, binned, stacked and compared to the seismic model. The model can then be changed to match the depth trace segments that will be stacked better, moved closer to their correct three-dimensional position and will compare better to the model. This patent uses a rather extensive seismic process to modify a seismic model that is not accurate.
One source of geologic exploration data that has not been used extensively in the past is potential fields data, such as gravity and magnetic data, both vector and tensor data and using potential fields data in combination with seismic data to provide a more accurate depth model or to derive a velocity model.
Gravity gradiometry has been in existence for many years although the more sophisticated versions have been held as military secret until recently. The measurement of gravity has become more acceptable in the late eighteen hundreds when measuring instruments with greater sensitivity were developed. Prior to this time, while gravity could be measured, variations in gravity caused by the effect of a large nearby object at one location, the gravity gradient, could not be reliably measured.
It has been known since the time of Sir Isaac Newton that bodies having mass exert a force on each other. The measurement of this force can identify large objects having a change in density even though the object is buried beneath the earth""s surface or in other ways out of sight.
Exploration for hydrocarbons in subsurface environments containing anomalous density variations such as salt formations, shale diapers and high pressure zones create havoc on seismic imaging techniques by concealing geologic structures beneath zones of anomalous density. By utilizing gravity, magnetic and tensor gravity field measurements along with a robust inversion process, these anomalous density zones can be modeled. The spatial resolution obtained from this process is normally much lower resolution than that obtained from acoustic seismic data. However, models obtained from gravity and magnetic data can provide a more accurate starting model for the seismic processing. Using the potential fields data models as a starting point for two dimensional and three dimensional seismic depth imaging greatly enhances the probability of mapping these concealed geologic structures beneath the zones of anomalous density.
The present invention provides a method and apparatus in which a gravity and magnetics inversion technique developed for use with gravity, Full Tensor Gradiometry (FTG) gravity and magnetics can be used as a driver for building earth models for seismic data processing. The result of the inversion process produces an earth model that contains the geometric description of three dimensional and two dimensional lithologic bodies with anomalous density and velocity. These geometries and the assigned velocities and densities can be used as an input to pre- and post-stack processing steps of seismic data that require a geometrically correct velocity model. Such methods include, but are not limited to Kirchhoff migration, Dip moveout (DMO), finite difference migration and f-x migration. The present invention entails the development of a very robust inversion process to produce models based on vector and tensor potential fields data, both gravity and magnetics. These data combined with seismically imaged portions of the structures as well as a three dimensional density model are input in the inversion process to image the overall anomalous formations. For example, in the case of salt formations in the Gulf of Mexico, a top of a salt map derived from seismic imaging along with a density model and bathymetry, are utilized to produce a base of salt model from the inversion process.
The present invention provides a method for determining parameters representing an anomalous subterranean formation. One or more components of potential fields vector and tensor data are measured at a plurality of locations over a region including the anomalous formation. The potential fields may be either gravity fields or magnetic fields, vector and/or tensor. A geophysical model of the region including the anomalous formation is determined. The seismic data gives a good model to the top of the anomalous formation. A value of the one or more components of the potential fields vector and tensor data at the plurality of locations is estimated for the model. A difference between the estimated values and the measured values at the plurality of locations is determined. The model of the region is updated based on the difference. The estimate of the value of the one or more components, the determination of the difference and the updating of the model is repeated until the difference reaches a minimum value. The updated model is used to determine the parameter of interest. The method is equally applicable to combined gravity and magnetic data. In the method of the present invention the subterranean formation may be selected from the group consisting of a salt body, a shale diapir, and extrusive or intrusive igneous bodies. In one embodiment the parameter of interest can be combined with seismic data representing the same parameter for depth imaging to provide a stacked depth model and to derive a velocity model. In another embodiment, seismic data can be introduced into the inversion process to further refine the lower boundary parameter predictions. The process of inversion followed by seismic imaging followed by another inversion and seismic imaging step is repeated until the results of the gravity magnetics inversion and the seismic imaging processes converge to a single answer or begin to diverge indicating that there is not a unique solution.